This invention relates to a high sensitivity optical signal receiver. More particularly, the invention relates to a method and system for receiving and converting optical signals with a high signal to noise ratio.
Optical receivers are used in fiber optical networks such as those for telecommunication networks in order to detect light signals. All optical receivers currently function as a single-ended threshold optical signal detector which uses a photo detector and a DC reference to produce a digital signal in response to an optical input signal. Input light pulses are sensed by a single photodetector that converts light energy into an electrical current. The current pulse is then sensed by either a transimpedance or high-impedance amplifier and converted into a voltage signal. The output of the amplifier is further filtered electrically into an output signal which enters a voltage comparator for logic level conversion.
The output of the comparator is a digital bit equivalent to the bit data represented by the input light signal. A comparator logic ONE output value equates to the presence of a light pulse while a logic ZERO equates to the absence of a light pulse. The output of the comparator represents the separation point between analog processing for the comparator input and digital processing of the output.
A typical optical telecommunications link consists of a transmitter light source, an optical fiber span, interconnecting optical elements and the receiver. The success of the receiver to determine the presence of light pulse depends on the available signal-to-noise ratio. In an optical transmission system, there are many variables that distort and contaminate light signals traveling in the fiber as well as noise levels at the receiver. Common optical signal degradation factors are laser output power limitations, fiber attenuations, splitter losses, excess termination losses, laser extinction ratio, in-line optical amplifier gain and detector quantum efficiency. Factors that will increase the noise factor are dark current noise, amplified spontaneous emission noise, crosstalk noise, modal noise, phase noise, laser noise, Johnson thermal noise, shot noise and electronic amplifier noise. In particular, Johnson thermal noise, shot noise and electronic amplifier noise are of the most concern for optical receivers.
Typically PiN photodiodes in conjunction with a load resistor are used for optical receivers because they are the only electrical circuit stable enough to run at multi-gigabit rates. The load resistor functions to quickly discharge the photodiode after the detection of a light pulse. However at high frequencies above 1 Ghz, Johnson noise from the load resistor is predominant. This noise may be 1,000 times higher than amplifier electronics noise and 10,000 times higher than shot noise. The load resistor value must be low to achieve a high bandwidth by having a short RC time constant which is governed by the resistor value and the internal photodiode capacitance. As the RC value decreases, the bandwidth of operation will increase. However, a low resistor value also generates higher Johnson noise resulting in a tradeoff between noise and discharge time.
Signal levels are always positive in polarity with respect to signal ground in optical signal detection. This method of detection is highly efficient when signal levels are strong since a signal pulse can easily be discerned using a DC threshold reference level that is substantially above background noise. The DC threshold reference level is ideally set at the mid-point between detection probability functions for a ONE and a ZERO. With weaker signals, setting the DC threshold level becomes increasingly difficult. This problem may be minimized by using automatic gain control or AGC. However, AGC requires an error signal before a correction shift may be made. The elapsed time between a transient error and the AGC response is a major limiting factor as a fast AGC response leads to instability problems while a slow AGC response limits its effectiveness.
To address these shortcomings, an approach to optical telecommunications transmission technology, that was patterned after superheterodyne radio receivers. This optical format required special modulation of the transmitted signal that altered either the amplitude, phase, frequency or polarization of the carrier light frequency. Data was not transmitted as simple on and off pulses but as continuous light. At the receiver, a strong monochromatic local laser at a specific wavelength is mixed with the weak input signal to produce an intermediate or IF frequency similar to a radio receiver. The IF frequency is then processed through IF filters to demodulate the encoded information into an amplitude signal. It finally enters a threshold circuit that converts the signal back to the original ONE and ZERO data stream. This method of data extraction is commonly known in linear circuits as phase lock loop demodulation. To accomplish the mixing in the optical domain, an evanescent coupler is used to mix the two signals (the local oscillator and the input light signal) to form two copies of the signal. Each copy of the light signal is sensed by separate photodiodes connected in a balanced detector arrangement that parallels a xe2x80x9cWheaton-bridgexe2x80x9d circuit. The teaching from this balanced detector arrangement was limited to common mode cancellation of local oscillator noise (laser spontaneous emission noise). Coherent detection has been replaced by simple direct-detection because of its complexity and incompatibility with dense wavelength division multiplexing (xe2x80x9cDWDMxe2x80x9d) solutions.
Thus, a need exists for an optical receiver which allows high bandwidth without significant delays due to high resistance. There is a further need for an optical receiver which allows both differentiation and integration of an optical signal conversion. There is also a need for an optical receiver with an efficient signal to noise ratio. There is also a need for an optical receiver with common mode rejection to allow improved dynamic range. There is additionally a need for an optical receiver which may be integrated with other processing electronics. There is also a need for an optical receiver which allows flexibility in components for biasing the electrical output.
These needs may be addressed by the present invention which is embodied in an optical signal receiver for reception of an optical signal and conversion of that signal to an electrical signal. The receiver has an optical amplifier capable of receiving the optical signal. An optical splitter is optically coupled to the optical amplifier and has two optical outputs. An optical sensor is coupled to the optical splitter and has a first output terminal and a second output terminal. A first photo detector which produces an electrical signal in response to a light input is coupled between the first and second output terminals. The first photo detector element is exposed to the first output of the optical splitter. A second photo detector which produces an electrical signal in response to a light input is coupled between the first and second output terminals and in parallel with the first photo detector. The second photo detector element is exposed to the second output of the optical amplifier. The signal from the second output of the optical splitter is delayed relative to the signal from the first output.
The invention may also be embodied in a method of receiving an optical signal and converting the signal to an electrical signal. The light signal is amplified and then split into a first and second segment. The first segment is delayed and the first segment of the light signal and the second segment of the light signal are converted into electrical signals. The electrical signals are compared to generate an electrical signal representative of the optical signal.
The invention may also be embodied in an optical receiver for converting an amplified optical signal on an optical fiber to an electrical signal. The receiver has an optical connector connected to the optical fiber and a passive substrate. An active substrate is mounted on the passive substrate. A splitter is fabricated on the active substrate and coupled to the optical connector, the splitter has two outputs for splitting the optical signal. A first and second waveguide are coupled to the two outputs of the splitter respectively, the first waveguide being longer than the second waveguide. A first photo detector is optically coupled to the first waveguide and has an anode and a cathode. A second photo detector is optically coupled to the second waveguide and has a cathode coupled to the anode of the first photo detector and an anode coupled to the cathode of the first photo detector. An output node is coupled to the anode of the first photo detector and the cathode of the second photo detector.
It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention.